Atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) is a widely used s-triazine (i.e., symmetric triazine) herbicide. Approximately 800 million pounds were used in the United States between 1980 and 1990. Numerous studies on the environmental fate of atrazine have shown that atrazine is a recalcitrant compound that is transformed to CO.sub.2 very slowly, if at all, under aerobic or anaerobic conditions. It has a water solubility of 33 mg/L at 27.degree. C. Its half-life, i.e., time required for half of the original concentration to dissipate, can vary from about 4 weeks to 57 weeks if in soils at low concentration, i.e., less than about 2 parts per million (ppm). High concentrations of atrazine, such as those occurring in spill sites have been reported to dissipate even more slowly.
As a result of its widespread use, atrazine is often detected in ground water and soils in concentrations exceeding the maximum contaminant level (MCL) of 3 .mu.g/L, i.e., 3 parts per billion (ppb), a regulatory level that took effect in 1992. Point source spills of atrazine have resulted in levels as high as 25 ppb in some wells. Levels of up to 40,000 mg/L, i.e., 40,000 parts per million (ppm) atrazine have been found in the soil of spill sites more than ten years after the spill incident. Such point source spills and subsequent runoff can cause crop damage and ground water contamination.
Current technology for reclaiming soil polluted with s-triazine compounds involves incineration or land farming of the polluted soil. Decontamination of ground-water containing s-triazine compounds involves pumping water to the surface and removing the pollutants on a sorbent. Similar technology is currently used for the decontamination of waste water from atrazine manufacturing plants. These are expensive methods and/or produce concentrated toxic materials that may present future hazards.
The persistence of atrazine in the environment has stimulated investigations into its biodegradation using bacterial cultures. The results have been of limited success, however. See, for example, Geller, Arch. Environ. Contam. and Toxicol., 9, 289-305 (1980); and Fernandez-Quintanilla et al., Proc. EWRS Symp., 301-308 (1981). Generally, less heavily substituted and nonchlorinated s-triazines are more biodegradable than is atrazine. For example, two soil fungi have been shown to degrade cyanuric acid, a nonchlorinated s-triazine, but not atrazine, to CO.sub.2. See, Wolf and Martin, J. Environ. Qual., 4, 134-139 (1975).
Certain bacteria (Pseudomonas and Klebsiella) have been isolated that are capable of utilizing specific s-triazine compounds, but not atrazine, as a sole nitrogen source. See, for example, Cook, FEMS Microbiol. Letters, 46, 93-116 (1987). Other Pseudomonas strains N-dealkylate atrazine and use the side chain carbons for supporting slow growth, but they do not degrade the s-triazine ring. See, for example, Behiki and Khan, J. Agric. Food Chem., 34, 746-749 (1986), and McMahon et al., Environ. Sci. Technol., 26, 1556-1559 (1992).
Other bacteria have been isolated that can degrade the ethyl or isopropyl side chains of atrazine; however, only N-dealkylation of the atrazine side chains typically occurs. Complete and rapid metabolism of the s-triazine ring or dechlorination has been demonstrated in very few situations. For example, a Nocardia sp. strain is capable of dealkylating and deaminating atrazine to form the unstable metabolite 4-amino-2-chloro-1,3,5-triazine, which is reported to undergo rapid chemical dechlorination followed by spontaneous ring cleavage. See, Giardi et al., Agric. Biol. Biochem., 49, 1551-1558 (1985). Recently, a very slow liberation of CO.sub.2 from the atrazine ring was observed in soil bioreactors. See, Nair et al., Environ. Sci. Technol., 26, 2298-2300 (1992). Less than 10% of uniform ring labeled [.sup.14 C]atrazine was converted to .sup.14 CO.sub.2 in 125 days.
Besides bacteria, the low specific activity of some enzymes involved in the degradation of atrazine metabolites from known microorganisms has proven to be too low for practical use in the degradation of s-triazine waste. See, for example, Cook, FEMS Microbiol. Rev., 46, 93-116 (1987). In sum, most known organisms exhibit slow rates of atrazine dealkylation and do not efficiently dechlorinate atrazine or destroy the s-triazine ring.
Thus, the use of microorganisms to completely degrade atrazine and less bulky s-triazine compounds is to date not commercially viable. However, no one has fully exploited the atrazine degradation ability of bacteria, nor in particular the enzymes of such bacteria, in order to both rapidly and completely degrade atrazine and related s-triazine compounds. Accordingly, there is a need for a method to rapidly and completely degrade atrazine and related s-triazine compounds by employing bacterial cells or by employing cell-free preparations from such bacteria induced to produce enzymes involved in the degradation process.